Electronic identification system

ABSTRACT

The invention relates to a method of identifying a plurality of transponders each of which transmits data at intervals to a receiver. The invention also relates to an identification system comprising a plurality of transponders and a receiver, to the transponders themselves and to an integrated circuit for use in a transponder. The transponder repeats data to the receiver at random or pseudo-random intervals in length. The intervals are directly or indirectly dependent on the output signal from a counter responsive to a physical characteristic of the transponder.

FIELD OF INVENTION

This invention relates to a method of identifying a plurality oftransponders each of which transmits data at intervals to a receiver.The invention also relates to an identification system comprising aplurality of transponders and a receiver, to the transponders themselvesand to an integrated circuit for use in a transponder.

BACKGROUND TO TH INVENTION

Identification systems are known in which a plurality of transmitters,typically transponders (commonly called tags), are activated by a powersignal (or an “interrogation signal”) and then transmit signals, usuallycontaining identification data to a receiver, which typically forms partof the interrogator. The signals may be transmitted in many ways,including electromagnetic energy, eg. radio frequency (RF), infra red(IR), coherent light and sound, eg. ultra-sound. For example thetransmission may be achieved by actual emission of RF energy by thetransponders, or by the modulation of the reflectivity of an antenna ofthe transponder, resulting in varying amounts of RF energy in theinterrogation signal being reflected or back-scattered from thetransponder antenna.

Radio Frequency Identification systems are used to remotely identify,take a census of, locate or otherwise interact with people, objects orgroups or clusters of people or objects. The systems usually compriseinterrogators also known as readers, and transponders also known astags.

It is not usually a problem for a reader to communicate with a singletag which is presented to the reader, such as in an access controlsystem. However in the situation where many tags may be present in areader's field of views, such as a crowd of people, or a pallet load ofgoods having tags attached, the transmissions by the tags would occurtogether and cause collisions, rendering the transmissions unusable dueto mutual interference. A number of arbitration methods have beendeveloped to enable a reader to sort and/or isolate and transact withthese large populations of tags. These methods are known variously asanti-collision schemes or collision-arbitration algorithms.

In one example described in U.S. Pat. No. 5,537,105 (corresponding to EP494114 B1) by Marsh et al, the whole contents of which are incorporatedherein, the transponders on receipt of an interrogation signalrepeatedly transmit a response signal containing data which identifiesthe transponder. The interrogator detects successful identification ofany transponder and briefly modifies the interrogation signal toindicate the successful identification. Each transponder includes alogic circuit responsive to a respective modification in theinterrogation signal to cease transmission of its own response signal.The response signals are transmitted at random intervals until theidentity of a transponder is successfully read and acknowledged by thereader and placed into a dormant or gagged state. U.S. Pat. No.5,699,066 (corresponding to EP0585132) and PCT application GB98/01385(corresponding to WO/985142) also describe methods in which the responsesignals are transmitted at pseudo-random intervals. The whole contentsof EP0585132 and WO/985142 are incorporated herein by reference.

Other examples of such methods are described in U.S. Pat. No. 5,699,096,Cole WO 01/41043 A1, and Maletsky U.S. Pat. No. 6,104,279.

Methods have been used to improve the randomness of the responseintervals. In EP 467036 B1, the whole contents of which are hereinincorporated, the identification system uses a pseudo-random delaybetween transponder data transmissions. In this example, a linearrecursive sequence generator is seeded by the transponder identificationaddress to provide the pseudo-random delay between tag datatransmissions. U.S. Pat. No. 5,550,547 describes a similar system inwhich the tag sends out a 64 bit ID code at intervals determined by arandom number generator. U.S. Pat. No. 6,104,279 describes a system inwhich remote units re-transmit their bit pattern at random intervals. Itfurther mentions that there are many techniques to produce a randomnumber; for example the identification number can provide the seed for arandom number generator permitting the user to individually seed eachtag with a different random number.

Another method is based on slotted polling or slotted Aloha schemes inwhich tags randomly select a time slot in which to transmit and thentransmit when it is their turn to do so. The theory is that becauseslots are randomly selected, sooner or later all tags will have had theopportunity to transmit messages ‘in the clear’. WO 01/41043 describessuch a system in which RFID tags randomly select a slot in which totransmit. In a practical implementation the slot selection by a tag ismade on a pseudo-random basis, using a seed for a random numbergenerator, which is derived from either part of the data held on the tagor by pre-programming a seed where the tag is manufactured. Thepossibility is great that many tags will have the same slot allocationchoice. The fewer the number of slots to choose from, the greater willbe the probability that many tags will ‘randomly’ select the same slottime and so will always collide and will therefore never be successfullyread.

All the systems described rely on the use of random or pseudo-randomtiming of the tag transmission or reply signal. The random numbergenerators (RNG) or pseudo-random number generators (PRNG) referred toin all the specifications above, rely on a recursive method ofgenerating a random number from a seed. Because of their nature, seededrandom number generators will always repeat the randomisation patternand the pattern will be fully predictable. If the recursive randomnumber generator had an infinite length, each seed would result in aunique pattern. However, RNG's have a finite length and in realapplications will be typically 10 to 16 bits long. It is probable thatin a cluster of 100 tags there will be several tags that will have asimilar or identical transmit repeat [slot] pattern. If these tags wereplaced in the reader field at the same time, they would repeatedlytransmit their identity together and hence would always clash and wouldnever be successfully read. The problem is that seeded random numbergenerators are not truly random (hence they are known as pseudo-random)and therefore do not provide the level of protection against clashesthat is required for the efficient operation of the collisionarbitration systems described in the cited patents.

In PRNG systems—the random pattern is only random for different seedvalues. The same seed value produces a predictable and consistentpattern. The finite length of an RNG will result in many tags havingexactly the same repeat pattern. Furthermore, no matter how long theseed, only a subset of the seed will actually influence the pattern. Theshorter the RNG shift register, the smaller the number of differentrandom numbers or patterns.

SUMMARY OF THE INVENTION

The present invention strives to overcome the disadvantages in the priorart and eliminate the aforesaid disadvantage of a pseudo-random slotselection or random transmit hold-off method.

The method of the present invention also strives to overcome theinherent problem of generating a random slot number or transmit hold offdelay in RFID tags—when using a seeded random number generator. A seededRNG is by its nature only pseudo-random in that for a given registerlength and tapped feedback points, a seed of a given value will alwaysyield the same pattern.

In one aspect of the present invention there is provided anidentification system comprising a reader including a transmitter fortransmitting a signal and a plurality of transponders, each transponderincluding a receiver for receiving the reader signal and a transmitterfor generating a response signal containing data which identifies thetransponder, the transponder being adapted to repeat the transmission ofthe response signal at intervals which are random or pseudo-random inlength, characterised by a circuit responsive to a physicalcharacteristic of the transponder, the intervals between the responsesignals being directly or indirectly dependent on the output signal fromsaid circuit.

The physical characteristic of the circuit, in a preferred embodimentpart of an RFID chip, provides a “true random result”, the output signalfrom the circuit affecting the randomness of the intervals between theresponse signals.

In a preferred embodiment said circuit comprises a counter driven by aclock, which may or may not be the same clock which used to drive thelogic of the chip, the output from the counter providing a randomnumber, or a random interval signal or providing a seed value for arandom number generator. The counter and the clock may be reset uponactivation of a POWER-ON-RESET (POR) circuit.

In another aspect of the invention there is provided a method ofidentifying a plurality of transponders comprising; exposing atransponder to RF whereby a capacitor is charged to a predeterminedvalue to activate a POWER-ON-RESET (POR) circuit, the transponder beingresponsive to a command signal from a reader to repeat the transmissionof a response signal, containing data which identifies the transponder,at intervals which are random or pseudo-random in length, characterisedby a circuit responsive to activation of the POR to provide an outputsignal when the command signal has been received, the output signalproviding a random number or a seed for a random number generator usedto determine a slot selection or random transmit repeat (hold-off) valuefor the response signals.

In a further aspect of the invention there is provided a transpondercomprising receiver means for receiving a reader signal, transmissionmeans for transmitting a response signal containing data whichidentifies the transponder, the transponder being adapted to repeat thetransmission of the response signal at intervals which are random orpseudo-random in length, characterised by a circuit responsive to aphysical characteristic of the transponder, the intervals between theresponse signals being directly or indirectly dependent on the outputsignal from said circuit.

In a further aspect of the invention there is provided an integratedcircuit for use in a transponder, comprising receiver means forreceiving a reader signal, transmission means for transmitting aresponse signal containing data which identifies the transponder, theintegrated circuit being adapted to repeat the transmission of theresponse signal at intervals which are random or pseudo-random inlength, characterised by a random circuit responsive to a physicalcharacteristic of the transponder, the intervals between the responsesignals being directly or indirectly dependent on the output signal fromsaid circuit. In a preferred embodiment the integrated circuit is a RFIDchip.

DESCRIPTION

The invention will be described further by way of example with referenceto the accompanying drawing, in which:

FIG. 1 is a block diagram showing a circuit for use in a transponderaccording to a first embodiment of the invention and

FIG. 2 shows a timing diagram illustrating the operation of fourtransponders of the present invention.

FIG. 3 shows a block diagram of a typical RFID chip incorporating thecircuit according to the first embodiment of the invention.

A transponder (tag) comprises an integrated circuit in the form of anRFID chip a part of which is shown in FIG. 1. When the RFID chip isexposed to an RF field or when voltage is applied—the chip goes througha POWER-ON-RESET (POR) sequence. When the recovered DC supply voltage isstable, the POR circuit provides a signal to the circuits on the chip,which signal initialises or resets the chips circuits. At this point theclock starts running and drives the counter. The output of the counteris routed to the circuits in the chip that require a random number.Examples of these circuits could be the slot selection described in WO01/41043, or the random transmit timer described in U.S. Pat. No.5,699,096 or U.S. Pat. No. 6,104,279. It could also for example be usedto derive the tag signature described in the international committeedraft standard CD ISO 18000-6 type A or the signature described inEP1001366A2

The instant, that POR occurs with respect to the application of RFpower, can vary substantially from chip to chip or from one power onsequence to the next, due to many factors. One of these factors is thedelay due to the time it takes for the DC power storage capacitor tocharge. Only when the capacitor has charged to a predetermined valuewill the POR circuit activate. This charging time is determined by anumber of mechanisms:—

-   1. The RF voltage applied to the chip-   2. The impedance of the antenna—in particular the series    resistance—often called the Radiation Resistance-   3. The size of the DC (VDD) storage capacitor-   4. The actual POR voltage threshold-   5. The leakage resistance in the circuit-   6. The impedance of the rectifier diodes

Referring to the timing diagram in FIG. 2, at some point in time afterthe RF has been applied to the chip, a reader will issue a command whichis received by all chips within range. It is probable that all chipswithin range will have started their clocks running at slightlydifferent times due to the charge times of their DC storage capacitorand due to the slightly different POR characteristics between tags.

When the chips receive the command (which could be a wake-up or othercommand etc) the value in the counter in each of the chips will bedifferent due to the slightly different start times. The value at theinstant of the command (command snapshot) is used as the random numberor as a seed for a random number generator used to determine the slotselection or random transmit repeat (hold-off) value. Therefore eachchip will have a different value to be used for slot allocation ortransmit hold-off time.

Furthermore if the counter in each and every chip continues to run andbecause no two counters will have the same clock period due to physicalvariances etc—the value in one chip's counter will be different (appearto be randomly different) with respect to the value in all other chipsat any point in time. The clock frequency of each chip clock will beslightly different due to supply voltage variations, chip manufacturingvariations, chip leakage etc. Therefore each time a command is receivedfrom a reader (causing a command snapshot) each chip will have adifferent value in their counter with respect to other chips andtherefore the value that is used to allocate a transmit slot or hold offdelay will be different for each chip and will be truly random.

It will be appreciated that the method described does not dictate theuse of a random number generator—but that the snapshot value of the freerunning counter could be used directly to allocate a slot in which thetag will transmit, or alternatively the value could be used to seed aRNG. It will also be appreciated that the method disclosed could also beused (this is the preferred embodiment) to randomly assign a transmithold-off period which need not conform to any slotting mechanism butwhich provides true randomisation of tag data transmissions. As the tagpopulation increases, so does the total time it takes to read apopulation of tags. This is beneficial using this method, because thelonger the free running counter continues to run, the more the countvalue will diverge from the counters in all other tags due to thenatural variation in clock frequencies due to chip manufacturer process,temperature, varying RF fields etc.

It will be further appreciated that the method taught, is not onlyapplicable to the randomisation of tag transmissions in RFID systems butthis method may also be used to provide any random number that may berequired in a tag circuit such as a session identity or tag signatureetc.

FIG. 3 shows the integration of the invention into an RFID chip. In thechip, the slot allocation circuit which could also be a transmithold-off and retry circuit, is driven by two control signals. A firstcontrol signal is a random input signal derived from the system clockand the randomising counter. A second control signal, called the commandsnapshot control, causes the slot allocation circuit to take a snapshotof the random input signal value, and uses this value to allocate a slotin which the tag will transmit alternatively will use this value as ahold-off delay value for the next transmission by the tag. Each time thetag receives a command, which may or may not necessarily be directed tothe tag itself, the command decoder generates a snapshot signal, therebycausing a fresh slot to be randomly selected or hold-off value to berandomly generated. It will therefore be evident to those skilled in theart, that the randomness of the method is entirely dependent on physicalcharacteristics which will be different from every other RFID chip ortag.

It will be seen that the invention efficiently attains the objects setforth above, among those made apparent from the preceding description.Since certain changes may be made in the above embodiments withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings be interpreted as illustrative and not in a limiting sense.

1. An identification system comprising a reader including a transmitterfor transmitting a signal and a plurality of transponders, eachtransponder including a receiver for receiving the reader signal and atransmitter for generating a response signal continuing data whichidentifies the transponder, the transponder being adapted to repeat thetransmission of the response signal at intervals which are random orpseudo-random in length, characterised by a counter driven by a clock,the output from the counter providing a random number or providing aseed value for a random number generator to affect the randomness of theintervals between the response signals.
 2. An identification system asclaimed in claim 1, wherein the counter and the clock are reset uponactivation of a POWER-ON-RESET (POR) circuit.
 3. An identificationsystem as claimed in claim 1, wherein the counter and clock is part ofan RFID chip.
 4. A transponder comprising receiver means for receiving areader signal, transmission mans for transmitting a response signalcontaining data which identifies the transponder, the transponder beingadapted to repeat the transmission of the response signal at intervalswhich are random or pseudo-random in length, characterised by a counterdriven by a clock, the output from the counter providing a random numberor providing a seed value for a random number generator to affect therandomness of the intervals between the response signals.
 5. Atransponder as claimed in claim 4, wherein the counter and the clock arereset upon activation of a POWER-ON-RESET (POR) circuit.
 6. Anintegrated circuit for use in a transponder, comprising receiver meansfor receiving a reader signal, transmission means for transmitting aresponse signal containing data which identifies the transponder, theintegrated circuit being adapted to repeat the transmission of theresponse signal at intervals which are random or pseudo-random inlength, characterised by a counter driven by a clock, the output fromthe counter providing a random number or providing a seed value for arandom number generator to affect the randomness of the intervalsbetween the response signals.
 7. An integrated circuit as claimed inclaim 6, wherein the counter and the clock are reset upon activation ofa POWER-ON-RESET (POR) circuit.
 8. An integrated circuit as claimed inclaim 7, wherein the integrated circuit is part of an RFID chip.
 9. Amethod of identifying a plurality of transponders, comprising exposing atransponder to RF whereby a capacitor is charged to a predeterminedvalue to activate a POWER-ON-RESET (POR) circuit, the transponder beingresponsive to a command signal from a reader to cause or repeat thetransmission of a response signal, containing data which identifies thetransponder, at intervals which are random or pseudo-random in length,characterised by a counter driven by a clock responsive to activation ofthe POR to provide an output signal when the command signal has beenreceived, the output signal providing a random number or a seed for arandom number generator, a slot selection or random transmit repeat(hold-off) value for the response signals being dependent directly orindirectly on said output signal.
 10. An identification system asclaimed in claim 1, wherein the counter and clock are routed to a latchsuch that when a command is received by the transponder, theinstantaneous value of the counter is stored in the latch.
 11. Anidentification system as claimed in claim 10, wherein the latch providesa random number or a seed value for a random number generator to affectthe randomness of the intervals between the response signals.
 12. Atransponder as claimed in claim 4, wherein the counter and clock arerouted to a latch such that when a command is received by thetransponder, the instantaneous value of the counter is stored in thelatch.
 13. A transponder as claimed in claim 12, wherein the latchprovides a random number or a seed value for a random number generatorto affect the randomness of the intervals between the response signals.14. An integrated circuit as claimed in claim 6, wherein the counter andclock are routed to a latch such that when a command is received by thetransponder, the instantaneous value of the counter is stored in thelatch.
 15. An integrated circuit as claimed in claim 14, wherein thelatch provides a random number or a seed value for a random numbergenerator to affect the randomness of the intervals between the responsesignals.